During the cold rolling process, the roller has a significant impact on key indicators such as the output of strip steel, plate surface quality, and cost per ton of steel. Defects in the roller during use may lead to a decline in product quality or even scrap, thereby reducing the yield rate and possibly causing equipment damage. Therefore, domestic cold rolling manufacturers prioritize roller research and strive to reduce roller consumption. This paper analyzes the causes of common roller defects in the cold rolling production process of stainless steel in a steel plant and proposes corresponding treatment measures.

Keywords

Cold rolling roller; Roller defect treatment; Roll grinding; Cold rolling production.

1. Analysis of chatter defects

During the cold rolling process of strip steel, a stripe with alternating light and dark and perpendicular to the direction of strip steel movement often appears on the surface of the strip steel. We call this defect “chatter.” According to actual production experience, the roller surface primarily causes the formation of chatter, which then transmits to the strip steel surface, forming similar defects.

1.1 Causes of roller vibration lines

The causes and manifestations of roller vibration lines mainly include the following:

The vibration of the grinding wheel spindle causes roller vibration lines. These vibration lines spiral across the roller surface, affecting the strip surface.

Poor grinding wheel shape or grinding wheel degranulation causes vibration lines. These lines also spiral across the roller surface. Usually, you can identify whether it’s a grinding wheel vibration line by comparing the roller speed and the grinding wheel speed.

Vibration lines caused by poor contact of the roller bracket: The vibration lines appear on the roller surface in the form of parallel form to the roller generatrix.

Poor shape of the tailstock tip causes vibration lines. These lines are parallel to the roller generatrix.

Poor rotation of the headstock causes vibration lines. These lines are also parallel to the roller generatrix.

The resonance of the grinding wheel headstock and the headstock causes vibration lines. These lines are also distributed along the roller generatrix.

1.2 Methods for eliminating roller vibration lines

To effectively eliminate roller vibration lines, you should first eliminate all abnormal vibration sources. You can select different grinding methods based on the degree of chatter marks. For obvious chatter marks, use coarse grinding to start the treatment; for less obvious chatter marks, start with semi-coarse grinding. Fine grinding can effectively reduce the impact of chatter marks and ensure the surface quality of rolls and strips.

Starting from the causes and elimination methods of chatter mark defects, this paper provides theoretical support for roll defect treatment in production and provides practical guidance for improving the quality and efficiency of cold rolling production.

2. Roller marks

1.The manifestation and causes of roller marks. Roller marks mainly manifest as pinholes, dents, indentations, and holes on the roller surface. The dents on the roller surface, which are generally circular and irregularly distributed, usually retain the surface state or texture of the roller. Debris (foreign matter) entering the bite area or the contact and friction between the rollers generally cause these marks. Preventive measures are: identify the source of debris and foreign matter, clean the rack, improve the process lubrication and cooling conditions, improve the surface quality and shearing quality of pickled coils, increase the surface hardness and hardening depth of the working rolls, and reasonably configure the hardness difference between the working rolls and the intermediate rolls.

2. Methods for eliminating roller marks. You remove roller marks by grinding or turning, depending on the degree of the marks.

3.knife marks

3.1 Causes of roll knife marks

Knife marks caused by mismatch of roll speed, grinding wheel speed, Z-axis longitudinal moving speed and grinding wheel feed; Knife marks caused by sharp edge or defect of grinding wheel edge; Knife marks caused by insufficient semi-finishing grinding pass and failure to eliminate defects of rough grinding pass; Knife marks caused by excessive load in finishing grinding pass; Knife marks caused by excessive roll convexity.

3.2 Preventive measures and elimination methods of roll knife marks

1. Preventive measures include configuring roll movement, Z-axis speed, grinding wheel feed, and speed. Key parameters are: a (grinding amount, mm), Vw (workpiece speed, m/min), Vs (grinding wheel speed, m/s), Ap (back cutting, mm), Ds (grinding wheel diameter, mm), and the number of effective grinding points. Chamfer the grinding wheel edge and perform tailstock grinding until defects disappear. Select standard grinding programs based on roll type and grind strictly. Ensure previous defects are eliminated before moving to the next program. Control the number of passes and load for finishing, select a softer grinding wheel, or reduce wheel convexity.

2. Methods for eliminating roller cutter flowers. Generally, repeated grinding is used to eliminate roller cutter flowers.

4.Cracks

1. Causes of roll cracks. When the roll is rolling or when the hot roll is grinding, cracks are easily generated due to local stress concentration; cracks are caused by accidents such as broken strips and twisted steel in the rolling mill; uneven thickness of incoming materials causes the roll to be impacted during rolling, causing excessive rolling force in a short period of time to cause cracks; fatigue cracks of the roll.

2.Methods for eliminating roll cracks. In order to prevent the roll from losing meat or causing larger defects due to cracks, it must be handled in time and ensure that the cracks are removed cleanly. During grinding, carefully check the roll. If cracks are found, mark the end of the roll neck to facilitate the next inspection. If the crack is small, the naked eye cannot determine whether the crack has been ground out after grinding for a period of time. At this time, an imaging test or flaw detection is required to determine whether the crack has been ground out. If it still exists, continue grinding; if the crack is large and deep, it is still obvious after a long period of grinding, and fixed-point cutting grinding can be performed to determine the depth of the crack.

5.Peeling (Flesh Falling)

Roll peeling refers to the phenomenon that a certain area of ​​the roll body separates from the roll body.

5.1 Roll surface peeling

1.During rolling, steel jamming or slipping caused by broken strip, tail swinging, overlapping steel strips, etc. causes thermal shock to the contact area of ​​the roll surface, causing varying degrees of burns on the roll surface, leading to stress concentration factors on the roll surface. If the rolling stress acting on the roll is greater than the tensile strength of the roll, surface cracks may diffuse during the rolling cycle and cause roll peeling.

2. High cycle fatigue peeling, this mode of subsurface cracking is generally more common in the second intermediate and support rolls, and usually occurs without a rolling mill accident. Generally speaking, the skin initiates this type of fatigue fracture, which is described as “brittle” peeling. The number of stress actions is very important. Even if the stress is lower than the strength of the roll material, it can cause cracks due to repeated actions, that is, fatigue cracks cause peeling. Since very small cracks distribute parallel to the tangent of the roll surface, high-cycle contact stress fatigue propagation can occur in many places within the stress area. Then repeated cyclic stress diffuses these cracks to the surface until peeling occurs.

5.2 Roller end peeling

Abnormalities in the thrust bearing of the rolling mill cause the roll end to peel. Incorrect or abnormal positioning of the rolling mill roll change trolley results in abnormal conditions during roll change, leading to roll end peeling. Hoisting or other unexpected conditions cause the roll end to suffer damage from impact.

5.3 Roller peeling treatment method

Roll surface peeling is treated by rough grinding + flaw detection until the defect is eliminated. Roller end peeling depends on the situation. Mild peeling is treated by grinding, and severe peeling causes the roll to be scrapped.

6.Supporting wall mark

1.Causes of roller support wall mark. Roller support marks generally exist on rolls with smaller diameters that require support from the roll body bracket during grinding. Rollers with support marks in cold rolling mills generally include the working rolls, one intermediate roll, and two intermediate rolls of the twenty-roll mill. Support marks are caused by burrs caused by uneven support, impurities in the support material, or grinding wheel abrasives, iron filings, and other debris mixed between the support and the roll, which scratch the surface of the roll during grinding.

2. Methods for eliminating roller support marks. Generally, support paper pads, half-pass grinding, or repeated grinding are used to eliminate support marks.

Contact：cykc@cykcgroup.com.cn

References:
[1] Ren Xilai. Failure Analysis and Repair of Cold Roll [J]. Steel Rolling, 2002, (3).
[2] Liu Defu, Yin Zhongda. Early Failure and Preventive Measures of work Roll in Cold Rolling [J]. Special Steel, 2003, (6).
[3] Ma Gang. Analysis and Preventive Measures of Roll Defects in Cold Rolling [J]. Shanghai Metal, 2007, (5).